Many fish were killed following the River Crane becoming poisoned overnight for the second time in three years.

Officers from the Environment Agency have been on the scene, working to stem the source of the pollution.

A spokesman confirmed the discharge had been stopped and work was ongoing to prevent further impacts.

People were dismayed to discover the water in the river turning black and many small dead fish floating on the surface this morning.

The source of the pollution has been traced to an outfall pipe upstream of the A30 but the substance itself has yet to be confirmed with tests ongoing.

Mark Heelis, of the Environment Agency, said: “Environment officers remain on site collecting evidence and sampling, and we have staff taking further samples from sections of the river and doing some ecological monitoring.

“Once the work to limit the pollution is complete, we will continue to monitor the river.

“We would like to thank members of the public for informing us of this pollution so that we could take prompt action to minimise the harm to this popular river.

“I can assure them that we are conducting a full investigation into the causes and impact of what has clearly been a serious incident.”

London Borough of Hounslow is advising dog walkers to keep their pets away from the river as a precaution.

The River Crane was polluted in October 2011 after a raw sewage spill killed thousands of fish.

It is estimated that 5km of the river, downstream towards the Thames, has been affected by the incident.

Multiple reports have been received of dead fish at locations along the affected stretch.

Water quality continues to be closely monitored.